The present invention relates to a ballast circuit for gas discharge tubes. More particularly, the present invention relates to a ballast circuit for gas discharge tubes having applied an alternating current (A.C.) or a direct current (D.C.) voltage source.
Recent improvements to the incandescent art have provided an improved lighting unit having a highly efficient gas discharge tube as the main light source and an incandescent filament as a supplementary light source. Such an improved incandescent lamp is generally described in U.S. Pat. No. 4,350,930, of Piel et al, issued Sept. 21, 1982, and which is assigned to the same assignee as the present invention.
The gas discharge tube has various modes of operation such as, (1) an initial high voltage breakdown mode, (2) a glow-to-arc transition mode, and (3) a steady state run mode. One of the circuit performance parameters is that the voltage applied across the gas discharge tube be such that the current flowing within the gas discharge tube is maintained above a critical value such as 60 milliamps. If the current flowing in the gas discharge tube drops below this critical value the arc condition of the gas discharge tube may extinguish, which, in turn, may cause the gas discharge tube to revert from its steady state run mode to its glow-to-arc transition mode or even to the initial breakdown mode. The reestablishment of the desired arc condition of the gas discharge tube may require a restrike voltage having a value typically 2.5 times or more than that of the operating voltage of the gas discharge tube.
The restrike voltage necessary for a gas discharge tube of 2.5 times its operational voltage presents a difficulty for a ballast circuit for a discharge tube operating directly from a 120 volt, 60 Hz A.C. source. For example, if the gas discharge tube has an operating voltage of 80 volts A.C. a restrike voltage of 80.times.2.5=200 volts or more is typically necessary and which voltage value is not ordinarily available from the peak-voltages of a typical 120 volt, 60 Hz A.C. source.
The gas discharge tube may be successfully operated by a ballast circuit developing a D.C. type operating voltage for the gas discharge tube. Such ballast circuits are as described in the previously mentioned U.S. Pat. No. 4,350,930, U.S. Pat. No. 4,320,325 of T. E. Anderson, issued Mar. 16, 1982, U.S. application Ser. No. 463,753, filed Feb. 4, 1983 of V. Roberts, and U.S. application Ser. No. 488,849, filed Apr. 26, 1983 of J. Davenport, all of which are assigned to the same assignee as the present invention.
The gas discharge tube may also be successfully operated by a ballast circuit operating directly from an A.C. voltage source and developing an A.C. operating voltage for the gas discharge tube. Such a ballast circuit is described in U.S. application Ser. No. 488,833, filed Apr. 26, 1983 of J. Davenport et al., and assigned to the same assignee of the present invention.
Although all of the hereinbefore mentioned ballast circuits will serve their desired function, it is desired that a ballast circuit be provided having applications to both direct current (D.C.) and alternating current (A.C.) supplied voltage.
Further it is desired that the ballast circuit supply only the desired amount of energy necessary to maintain the arc condition of the gas discharge tube.
Still further, it is desired that the ballast circuit, applicable to both D.C. and A.C. applications, be provided with means effective to easily adapt the ballast circuit to various needs of the arc discharge tube.
Accordingly, objects of the present invention are (1) provide ballast circuits applicable to both A.C. and D.C. applied voltages, (2) provide ballast circuits that supply only the desired amount of energy to the gas discharge tube which is necessary to maintain the arc condition of the gas discharge tube, and (3) provide the ballast circuits with means effective to be easily adaptable to various needs of the arc discharge tube.